The chargeable subscription starts automatically on completion of the free trial, in order to provide continuous uninterrupted experience. So as per the Apple website, if you do not want to continue using, you need to cancel the trial before you are charged. 

The trial is an introductory offer and is availed through AppStore using your Apple Account. A user can avail the trial only once. The trial and subscription are managed through Apple.

In case you opened this accidentally but want to know a similar answer for Android, here is the article.